For the production of fiber reinforced flat gaskets, which are required in many applications in the technical field, there is used a so called packing sheet calender which has a heated cylinder on which the sheets are rolled and vulcanized, and a cooled cylinder which serves as a pressing roller for the composition and is usually pressed hydraulically. The gaskets are thereby produced from rubber made workable by a solvent and fiber which gives the finished product its strength. Traditionally asbestos fiber was used as the reinforcing fiber, as the gasket production and use excluded other fibers on thermal grounds. However asbestos has become undesirable to an increasing extent on the ground of danger to health and in recent times is replaced, in-so-far as possible, by new high strength and thermal resistant synthetic fiber.
The production of satisfactory packing sheets depends on the precision of the roundness of the cylinders, the qualities of the cylinder surfaces, the temperature and its exactness, the uniformity of the roller pressure and, above all, the high precision of the circumferential velocity of the two cylinders (synchronism). With the traditional material of the packing sheets, thus with asbestos fiber reinforcement, it was sufficient, with respect to the synchronism of the cylinders, to use precision coupling wheels for the two cylinders. However there is a problem that with the coupling wheels, a practical exact synchronism can be obtained only with a definite sheet thickness. At the beginning of the sheet construction or upon exceeding these definite values, a deviation from the optimum occurs. The diameter of the heated cylinder changes with changes in temperature and also changes, in the life of the cylinder, when the cylinder must be reground on account of wear. For these reasons, the synchronism of the rollers use of coupling wheels to achieve is, in principle, insufficient i.e. limited.
Through use in the calender construction of known individual cylinder drives it has been sought to remedy this deficiency by providing precise mechanical control of the drive of the pressing cylinder as well as also using synchronous motors. However there remained the deficiency that it was practically impossible, in both cases, to attain the desired synchronism of the cylinders. A particular disadvantage lay therein that the optimal synchronism depends on the thickness of the sheet being formed, which in each process requires constant readjustment.
With the introduction of new fibers, which are not asbestos, these problems become even more serious. When, for example, the optimal adjustment of the synchronism is not precisely attained, there is a danger that the not yet vulcanized sheet being formed may be pulled apart and thereby destroyed.